It is known that the growing demand for portable electronic applications has caused an intense and strong interest of the experts in the field towards the development of alternative sources of high-density electric power such as Micro Fuel Cells, suitable for substituting traditional batteries with lithium ions.
Micro Fuel Cells that will be hereafter called micro cells, are devices that allow an easy, clean obtainment of electric power, with high performances. Micro cells exploit the energetic content of a particular chemical fuel, such as for example hydrogen or methanol, and through oxide-reduction reactions they produce electric power, in continuous current, supplying as reaction by-products: heat and water. Micro cells are thus very desirable also from their viewpoint of the low environmental impact.
A micro cell supplied with hydrogen is shown in FIG. 1A and comprises:                an anode A supplied with gaseous hydrogen H2 as a reactant and here, by means of a catalyst, it is separated into protons and electrons;        a cathode C supplied with oxygen as a reactant;        an electrolyte that separates the anode A and the cathode C which, in the present embodiment, instead of being liquid is solid and is made of a polymeric membrane PEM (“Proton Exchange Membrane”) being sandwich-wise arranged between two carbon layers serving as electrodes so as to form a thin membrane MEA (Membrane Electrode Assembly).        
The membrane PEM has the characteristic of easily absorbing the water, of not allowing the passage of the gases, thus maintaining hydrogen and oxygen separated from each other, and of being a conductor of ions but not of electrons. The electrons thus pass through an external circuit to the micro cell ensuring a continuous electric current suitable for supplying a load.
The carbon layers have a portion of a front layer that serves as a diffusion layer of the gas or GDL (Gas Diffusion Layer) and that comprises prearranged and suitable serpentines of outflow channels of the gaseous hydrogen H2, as shown in FIG. 1B.
The electric power in continuous current produced by the micro cell is a function of the active area of the MEA and of the amount of reactants introduced into the anode A and the cathode C. The oxide-reduction reactions for the generation of the electric power to the anode A and to the cathode C, with a suitable catalyst being present, typically platinum Pt, are the following:
To the anode A: H2→2H++2e−
To the cathode C: ½ O2+2H++2e−→H2O
Globally: H2+½ O2→H2O
In some typologies of micro cells, called the “passive type” (passive mode) or “air-breathing”, the oxygen supplied to the cathode C is provided in a natural way by the environment air surrounding the micro cell itself, while the gaseous hydrogen H2 may be stored as compressed gas in high pressure cylinders, or as liquid hydrogen in suitable cryogenic containers, or in the solid state in metallic hydrides or in the liquid state in chemical hydrides or in other materials able to absorb it in considerable amount, such as carbon grounds.
To enhance the voltage to be supplied to the load, systems are provided with two or more micro cells coupled in series to define a so called “stack” system that has a geometric development vertical with respect to the sliding surface of the reactants in each micro cell, as shown in FIG. 2. This implementation has the anode A and the cathode C of each micro cell, realized by means of plates of a material that has been suitably treated on a surface and being treated mechanically for the realization of serpentines of outflow channels of the gaseous reactants.
These “stack” systems require the use of intermediate plates of the bipolar type, that simultaneously serve as anode A on one side and as cathode C on the other for two distinct consecutive micro cells of the stack, which allows separating the gaseous reactants. The plates at the ends of the “stack” system are instead of the monopolar type serving exclusively as anode A or as cathode C for the respective micro cell.
The known implementations of “stack” systems, although advantageous in several aspects and functional for the enhancement of the voltage, however, have some drawbacks. In fact, the gaseous reactants typically must be sent to a higher pressure than the atmospheric one for overcoming the fluid-dynamic resistances due to the channel serpentines realized in the bipolar plates. It is thus sometimes necessary to adopt suitable pump systems both for the hydrogen and for the oxygen. The pump systems may require energy for operating and a further control system to manage the amount of the reactants, and are also usually noisy. These “stack” systems are thus more complex from the constructive viewpoint, especially for the management of the oxygen.
Moreover, the vertical geometric development of the “stack” system may be unusable in some electronic applications and thus offer little versatility.
Moreover, the intermediate plates may require the use of a substrate with a high strength and with high thickness and weight to allow the mechanical treatment on the two faces; but this, however, may jeopardize the final performance of the system in terms of volumetric or gravitational power density. Alternatively, the use of a substrate with high mechanical resistance and with contained weight, such as for example graphite, has a high cost that may preclude the possibility of realizing “stack” systems being compact and low cost at the same time.
To solve these drawbacks, other solutions of systems for the generation of electric power with micro fuel cells employ plates or current collectors with compound material used for the realization of printed circuits PCB or Printed Circuit Boards. This may allow reducing the vertical dimensions in the case of the “stack” configuration and also may allow isolating each electrode in a coplanar configuration of the micro cells, allowing an easier parallel connection of the micro cells that may allow enhancing the current supplied to the load. An implementation is shown in FIGS. 3A and 3B.
The plates realized in PCB technology comprise one or more thin layers of non-conductive compound material, for example FR4 (Frame Retardant of the 4 type) and CEM1 (Composite Epoxy Material of the 1 type), covered by a layer of conductive material whereon it is possible to realize conductive tracks or passages for conveying the current generated. Thanks to the maturity of the PCB technology, it is also possible to integrate, through metallic tracks of copper, possible electronic circuits for the control and the conditioning of the electric power produced by the micro cells, thus realizing a compact and efficient system (System on a Package).
It is noted that the electric power supplied by the micro cells does not necessarily adapt itself to the electric power requested by a user/load and this implies the presence of conditioning circuits for adapting the electric power generated to the user's/load's requests. The ensemble constituted by the conditioning circuit of the electric power and by the final load, affect the connection mode in series and/or in parallel between the micro cells of the system.
The “stack” or planar system realized with PCB technology may have advantages and may allow realizing passive systems at the cathode, without the need of employing fans or compressors for the inflow of the oxygen from the air realizing planar “stack” systems of small power (1-5 W) that adapt themselves to the form factors of the portable electronic applications.
However, this type of system also has some drawbacks linked to the realization of systems with a request of an electric power to the load comprised between 5 and 10 W.
In fact, in a planar “stack” system of micro fuel cells in PCB technology, the implementation occupies a rather large surface area, and this may require a significant amount of material employed with a consequent problem of space for the integration in the portable electronic systems.